1. Field of the Invention
The present invention relates to printed circuit boards for usage in electronic systems such as computer systems. More specifically, the present invention relates to capacitive plane printed circuit board (PC-PCB) structures for suppressing electromagnetic fields and stabilizing integrated circuits.
2. Description of the Related Art
Multiple layer printed circuit boards (PCBs) are used in computer systems for interconnecting integrated circuit (IC) chips and other electronic components and devices. A multiple layered printed circuit board is formed from a substrate supporting a plurality of insulated conductive trace layers. The insulated trace layers typically include surface conductive trace layers and embedded trace layers with selected trace layers connected as a ground plane and a power plane. Integrated circuits and electronic components and devices are mounted on an outer surface of the multiple layered printed circuit board and selectively connected to the trace layers by plated-through holes called vias.
A problem arising with multiple layered printed circuit boards is that the electronic operation of integrated circuits includes switching that results in high frequency fluctuations in the potential difference between the power plane and the ground plane. This problem has been addressed using bypass capacitors connected between the power plane and the ground plane and mounted in the general vicinity of each integrated circuit. The bypass capacitors are generally effective in reducing and stabilizing voltage fluctuations for low frequency voltage oscillations below about 200 MHz. Unfortunately, a via connection of a bypass capacitor to the power and ground planes introduces a small inductance that impedes the bypass function of a bypass capacitor and reduces the effectiveness of the bypass capacitor to stabilize voltage fluctuations at higher frequencies. The poor high frequency performance of bypass capacitors is particularly unsuited for today's integrated circuits that have higher EMI spectrum and power plane current demands in the Very High Frequency (VHF) band range above 200 MHz. Conventional decoupling components such as through-hole and surface mount technology capacitors have an effective range of frequencies that is considerably below the 200 MHz range.
Other characteristics of recent-technology integrated circuits render bypass capacitors unsuitable for usage with recent technology. New integrated circuits are clocked at higher rates as illustrated by the rapid ascension of operating frequency of microprocessors. The size of integrated circuits is decreasing and, as die size decreases, the spectral content of operating circuits increases, escalating the VHF current demand. In addition, an increasing percentage of components in recent synchronous computer designs operate at the high clock speeds of processors using connection to high-speed buses such as the Intel PCI bus and the VESA VL bus. These characteristics create very high speed current demands in many geometric locations of a circuit board or printed circuit board (PCB).
Various structures and techniques for filtering or decoupling voltage fluctuations have been used to improve electromagnetic field suppression and stabilize voltages in high speed circuits. Previously the electronics industry has used traditional industry filtering or choking of power or signals in integrated circuits. Choking coils utilize an inductor to present a relatively high impedance to an alternating current to reduce the amount of voltage fluctuation in circuits. Ferrite cores are structures constructed from low-density ceramic materials having a very low eddy-current loss for reducing voltage fluctuations in circuits. Decoupling capacitors and various filters including RC, RCR and RCL circuits are liberally used in high numbers in traditional or printed circuit boards. The traditional structures and techniques have severe functional limitations and limitations of theoretical physics as printed circuit board speed increases.
Problems with the performance of bypass capacitors, chokes, filters and decouplers have been addressed using capacitive plane (CP) structures to control voltage fluctuations and improve electromagnetic field suppression using the physical and electronic theory of Planar Mutual Inductive Coupling (PMIC). The capacitive plane design approach allows high speed VHF energy to flow back and forth across the plane with minimum high frequency resistance or phase imbalance, thereby creating less waste. Electromagnetic fields are caused by this high frequency energy waste and phase imbalance so that reduction of waste suppresses EMI.
The disadvantages of bypass capacitors are addressed by Sisler, J. in U.S. Pat. No. 5,010,641, entitled "Method of Making Multilayer Printed Circuit Board", issued Apr. 30, 1991 by making a multilayer printed circuit board with sufficient internal distributed capacitance to eliminate the need for a bypass capacitor mounted in the vicinity of each integrated circuit mounted to the board. The method includes providing one or more fully-cured power-ground plane sandwich components with are laminated together with other partially cured component layers of the board. Each sandwich component includes a conductive power plane layer and a conductive ground plane layer separated by a fully cured dielectric material. The thickness of the dielectric layer of each sandwich is chosen to be sufficiently thin to supply a desired distributed capacitance.
What is needed is a structure and technique for improving electromagnetic field suppression and stabilizing voltages applied to integrated circuits mounted on a multiple layered, capacitive plane printed circuit board. What is further needed is a system and method for VHF current delivery and return to meet system design requirements for highspeed circuits such as microprocessor circuits.